Michigan researchers achieve quantum entanglement of three electrons

ANN ARBOR, Mich.The quantum entanglement of three electrons,
using an ultrafast optical pulse and a quantum well of a magnetic
semiconductor material, has been demonstrated in a laboratory at
the University of Michigan, marking another step toward the realization
of a practical quantum computer. While several experiments in recent
years have succeeded in entangling pairs of particles, few researchers
have managed to correlate three or more particles in a predictable
fashion.

The results were presented in an article on Nature Materials'
web site on February 23 and will appear in the March 4 issue of
Nature Materials, titled "Optically induced multispin entanglement
in a semiconductor quantum well." Authors of the paper are
Jiming Bao, Andrea V. Bragas, Jacek K. Furdyna (University of Notre
Dame), and Roberto Merlin.

Entanglement, which is essential to the creation of a quantum computer,
is one of the mysterious properties of quantum mechanics that contradicts
the notions of classical realism. Quantum computers will be able
to perform highly complex tasks that would be impossible for a classical
computer, at great speed.

Briefly, entanglement describes a particular state of a set of
particles of energy or matter for which correlations exist, so that
the particles affect each other regardless of how far apart they
are. Einstein called it "spooky action at a distance."
We know that we must be able to harness entanglement in order to
develop the quantum gates necessary for storing and processing information
in practical quantum computers. These devices will offer enormously
enhanced computing power that would permit extremely fast ways to
solve certain mathematical problems, such as the factorization of
large numbers.

The Michigan team, which has been working on the problem for several
years, used ultrafast (50-100 femtosecond) laser pulses and coherent
techniques to create and control spin-entangled states in a set
of non-interacting electrons bound to donors in a CdTe quantum well.
The method, which relies on the exchange interaction between localized
excitons and paramagnetic impurities, could in principle be used
to entangle an arbitrarily large number of spins.

In the presence of an external magnetic field, a resonant laser
pulse creates localized excitons (bound electron-hole pairs) of
radius ~ 0.005 microns in the CdTe well. Electrons bound to donor
impurities within that radius feel the presence of the exciton in
such a way that they became entangled after the exciton is gone.
The process involves resonant Raman transitions between Zeeman split
spin states. In the experiments, the signature of entanglement involving
m electrons is the detection of the mth-harmonic of the fundamental
Zeeman frequency in the differential reflectivity data.

"The community is trying various approaches
to achieve controllable interactions between qubits. We've
seen a variety of proposed solutions from atomic physicists involving
trapped ions and atoms and even 'flying qubits' based
on light," said Merlin. "Solutions based on semiconductor
technology, like ours for example, may well hold more promise for
practical implementation when combined with advances in nanotechnology."

The experiments have so far involved a large ensemble of sets of
3 electrons. "Our procedure is potentially set-specific and
scalable, which means that it shows definite promise for quantum
computing applications," Merlin said. Cryptography is expected
to be one of the first such applications.

The research was conducted at OPIL (Optical Physics Interdisciplinary
Laboratory), a laboratory of the FOCUS (Frontiers in Optical Coherent
and Ultrafast Science) Center of the University of Michigan and
funded by ACS Petroleum Research Fund, NSF (National Science Foundation)
and the AFOSR (Air Force Office of Scientific Research) through
the MURI (Multidisciplinary University Research Initiative) program.